Modern motor vehicles, particularly electric vehicles and hybrid vehicles, for the most part, have two on-board power networks, namely a low-voltage power system, for example of 12 V, and a high-voltage power system, the supply voltage of which is higher than that of the low-voltage power system. The low-voltage power system is supplied by a low-voltage battery, for example a 12 V lead battery (and optionally other energy sources such as an alternator), whereas its own high-voltage battery is provided in the high-voltage power system, which can be charged in a different way. If an electric motor is present, it can function as a generator and charges the high-voltage battery via the power connections thereof. In the case of electric vehicles and in the case of so-called plug-in hybrid vehicles, a charging device is provided, which permits charging up the high-voltage battery also from an electrical energy source that is external to the motor vehicle, for example, at a charging station.
Since the high voltage frequently exceeds the contact safety voltage, safety measures are necessary in order to be able to produce an absence of voltage in the high-voltage power system of the motor vehicle. A portion of these measures include first safety contactors that are provided within the high-voltage battery and that can be connected via a pilot line, for example, as is basically known in the prior art. Of course, a control device may also be present, for example, as a part of a battery management system or to create such a system, by way of which the first safety contactors can be closed or engaged and opened or disengaged. A specified power is usually necessary in order to keep the first safety contactors engaged, since these contactors must satisfy high requirements. In order to increase safety, a passive discharge resistor that is connected for the most part external to the battery between the positive and the negative power connections is also required by law in some countries.
It has already been proposed to permit an exchange of energy between the low-voltage power system and the high-voltage power system, so that it is also conceivable, in particular, to charge the high-voltage battery via energy sources of the low-voltage power system. For this purpose, a d.c. voltage transformer, in particular a bidirectional d.c. voltage transformer, which makes possible this energy exchange, can be connected between the low-voltage power system and the high-voltage power system.
It has also recently been proposed to integrate solar devices in motor vehicles, in particular as a sun roof, as another electrical energy source in motor vehicles. Such solar devices have at least one solar cell, which permits it to transform sunlight into electrical energy. During operation of the motor vehicle, the solar device can support operation in the low-voltage power system; when the motor vehicle is not in operation, i.e., in a non-operating state or a non-operating phase, it has been proposed to use the energy obtained from the solar device also for charging the low-voltage battery. Of course, low-voltage batteries, particularly the usual 12 V lead batteries, do not have a very large storage capacity, so that the total amount of energy that the solar device produces cannot be stored, in particular with longer down times of the motor vehicle. On the other hand, however, the power available from the solar device is rather small, so that a charging of the high-voltage battery by way of the power connections and a d.c. voltage transformer designed for supplying the low-voltage power system from the solar device has only a very low efficiency, if it is possible at all, for example, since the first safety contactors would have to be kept open or disengaged, and if need be, additional different components would have to be actively operating. Further, the mentioned passive discharge resistor represents a load via the power connections of the high-voltage battery during the charging process. Another problem relative to the charging of the high-voltage battery from the solar device in today's motor vehicles is that the high-voltage power system would still be under voltage even in a non-operating phase of the motor vehicle, which is not desired.